Audio transformer



1952 A. B. BERESKIN 3,061,804

- AUDIO TRANSFORMER Original Filed June 30, 1954 5 Sheets-Sheet 1 OUTPUTINVENTOR ALEXANDER B. BERESKIN AGENT INPUT? 1962 I A. B. BERESKIN ,804

AUDIO TRANSFORMER Original Filed June 30, 1954 5 Sheets-Sheet 2 98 2oilool //99 IOI am 0:0; M m

A U I I I I I l O U! n 2 v INVENTOR ALEXANDER B. BERESKIN N AGENT Oct.30, 1962 A. B. BERESKIN AUDIO TRANSFORMER 5 Sheets-Sheet 3 OriginalFiled June 30, 1954 INVENTOR ALEXANDER B. BERESK: N

AGENT Oct. 30, 1962 A. B. BERESKIN AUDIO TRANSFORMER 5 Sheets-Sheet 4Original Filed June 30, 1954 INVENTOR ALEXANDER B. BERESKIN AGENT Oct.30, 1962 Original Filed June 50, 1954 A. B. BERESKIN AUDIO TRANSFORMER 5Sheets-Sheet 5 INPUT HELD ALEXANDER I00 200 400 I000 2000 4000 FREQUENCY-CYCLES PER SECOND 1 5 INVENTOR B. BERE SKIN AGEN United States Patent01 3,061,804 AUDIO TRANSFORMER Alexander B. Bereskin, Cincinnati, Ohio,assignor to The Baldwin Piano Company, Cincinnati, Ohio, a corporationof Ohio Original application June 30, 1954, Ser. No. 440,505, now PatentNo. 2,924,780, dated Feb. 9, 1960. Divided and this application Feb. 1,1956, Ser. No. 562,843

7 Claims. (Cl. 336-84) This application is a division of my co-pendingapplication Serial No. 440,505, filed June 30, 1954, and then titledAudio Amplifier System.

The present invention relates generally to power amplifiers, and moreparticularly to wide band power amplifiers capable of utilization ashigh fidelity audio amplifiers.

The provision of amplifiers capable of amplifying a wide band of audiosignals with minimum distortion at high power levels is of considerableimportance commercially. The size and cost of a power amplifier togetherwith its driver and power supply, are primarily a function of theefficiency of the power amplifier, and of its power sensitivity, whenconsidered in relation to any specific audio power output. For a givenamplifier tube type, which allows a given plate power dissipation, ofthe order of four times as much power output may be obtained, operatingtwo tubes push-pull class B, as against class A. Therefore, while theoperation of high fidelity amplifiers class B presents serious problems,since such amplifiers are prone to introduce distortion, the increase ofefficiency and power output possible in this manner, when utilizing aspecific tube type, provides a strong incentive to their development. Ageneral technique for overcoming distortion is of course available, i.e.the introduction of negative feed-back into the amplifier system. Theadvantage of high efficiency operation also is reflected in the size ofpower supplies required to operate at a given power output, andtherefore the cost per watt of output may be reduced both at the powersupply and in the tube complement.

In order to obtain highest efficiency together with increased powersensitivity, the push-pull class B high fidelity amplifier maypreferably employ tetrode vacuum tubes Such tubes have higher outputimpedance than triodes, but the use of suitable feed-back compensatesfor the disadvantage. However, any tube type may be employed, and I donot desire to be limited to any specific type of tube in the practice ofthe present invention.

One of the major problems associated with class B operation is thatwhich arises due to energy storage in the leakage reactance existentbetween primary windings of conventional type. A. P. Sah, in an articleentitled Quasi-Transients in Class B Audio-Frequency Push-PullAmplifiers, Proceedings of the Institute of Radio Engineers, vol. 24,November 1936 pps. 19221541, has shown that the energy stored in thisleakage reactance gives rise to a discontiuity of conduction at thepoint of transfer of current conduction from one tube to the other of apush-pull pair. This discontinuity is sometimes denominated a conductiontransfer notch, and must be eliminated if class B operation is to besuccessfully employed, in high fidelity amplifiers. One solution residesin the elimination of leakage reactance, and various winding schemeshave been suggested which accomplish this end with more or less success.Use of bi-filar windings for the two primary halves has been highlysuccessful in this regard.

The use of bi-filar windings introduces problems in output transformerdesign. *One of these is that high voltage may exist between adjacentwires of the bi-filar windings,

3,061,804 Patented Oct. 30, 1962 ice and the wires must be adequatelyinsulated to withstand the voltage. However, adequately insulated wireexists commercially, so that this problem is not insurmountable. A moresignificant and difficult problem is that considerable capacitanceexists between the adjacent wires of a bi-filar winding, and that thecapacitance must be charged in developing voltage difference between thewires, i.e. a voltage cannot exist without the requisite charge. Thecharging current must be supplied by the tubes of the output stage, andthis necessity represents a major factor in limiting the high frequencypower-delivering capacity of the amplifier. For example, in one specificdesign of a bi-filar transformer a capacitance of .045 microfarad wasfound to exist between primary windings, which'leads', at a peak voltageof 500 volts cross the primaries, and at a frequency of 10 kc., to apeak charging current of 1.5 amperes, which may be beyond the capacityof the tubes employed.

A natural step to consider is the possibility that an inter-connectionof the primary windings may be found which will enable a reduction ofthis charging current. The possibility must be envisaged, however, inconjunction with the further requirement that the cathodes of the powertubes are to be maintained at ground potential, in accordance with thepresent invention. It is found that sectionalization and reconnection ofthe primary windings does not reduce the charging current, and generallyincreases the voltage across some parts of the windings to compensatefor a decrease at other parts, which increases the burden on theinsulation. It would therefore appear that the solution, insofar as theamplifier of the present invention is concerned, must be met by suitablydesigning the output transformer, rather than by sectiona'lization andreconnection of the sections.

In general, the capacitance between two isolated parallel wiresdecreases radically with increase of spacing therebetween. In atransformer winding each wire has capacitance with respect to two wireson each side thereof, in the same layer, and also with respect to wiresin the layers above it and below it. Effective capacitance between wiresin the same layer may be cut in half by transposing the two wires of thebi-filar pair at every turn. Capacitance between wires in adjacentlayers is not modified by this process, but since the capacitancebetween wires in the same layer accounts for two-thirds of the totaltransformer capacitance, the process does reduce the total transformercapacitance by one-third. As an alternative the windings may be randomwound.

The capacitance between adjacent layers of windings may be reduced byincreasing the spacing between the layers. This increases the leakagereactance of the transformer, but a balance may be attained ofcapacitance reduction with leakage reactance increase, by virtue ofwhich capacitance may be decreased to a significant extent withoutintroducing the undesirable conduction transfer notch to a noticeableextent. Obviously, definite limits exist in respect to the possibilitiesof this expedient.

The total number of turns and the core size may be reduced by the use ofgrained core material. This pro cedure enables inter-winding capacity tobe further reduced, and overall the total reduction made possible byutilization of the various recited features of the invention haveenabled a capacitance of .0 1 microfarad to be attained, which iscapable of operating at 60 watts output, with a peak to peak voltage ofabout 500 volts.

It is known to use bi-filar output transformers in push pull poweramplifiers operating class B. In the circuit of the present invention,however, and in distinction to known circuits of this type, the powertubes are operated with cathodes at a fixed reference potential, whichusually is ground. No bias resistances, nor by-pass condensers,

are present in the cathode circuits of the power tubes. This enables useof any combination of plate supply and screen voltage, and thereforesimplifies the requisite power supply. There remains, however, theproblem of proyiding suitable bias for class B operation. This problemis solved by employing a suitable driver circuit.

' The driver circuit employed in conjunction with the grounded cathodeclass B amplifier of the present invention is a phase inverter employingtwo triodes having their cathodes connected together and via a commoncathode resistance to a source of relatively high negative voltage. Theanodes are supplied in parallel, via separate anode load resistances, toa positive source of relatively low voltage, which may be derivedultimately from the relatively high voltage supply for the screen gridsof the power tubes, by means of a voltage divider connected from thescreen grid to ground. The current drain on the voltage divider due tothe phase splitter load is relatively constant, since the tubes of thephase splitter operate class A in push-pull relation. -It follows thatrelatively constant voltage is available for the anodes of the phasesplitter. One grid of the phase splitter is maintained at fixedpotential with respect to ground, and the other driven from anunbalanced source of signal, the circuit parameters being so selectedthat class A operation is attained, and push-pull signal output isderivable from the anodes of the tubes. These anodes are, at the sametime, at a DC. potential selected so that the control grids of the powertubes may be connected directly thereto, and when so connected willoperate class B, or adjacent thereto, and will never become sufficientlypositive in response todriving signal that appreciable grid current willflow. Since the driver circuit 'as seen by the control electrodes of thepower tubes is inherently a high resistance circuit, bias variation dueto grid current flow is avoided, and grid current flow itself radicallyreduced. The use of a direct coupled driver possesses the advantage ofsimplicity of power circuitry, but still further, the advantage thatcoupling capacitors are eliminated from the circuit, which eliminatesthe possibility of instability of the system and of blocking of thepower amplifier in response to excessive input signal, as "well as thepossibility of poor transient response due to coupling circuit timeconstants.

Negative feed-back is provided in the present system by means of asupplementary statically shielded secondary .winding of the outputtransformer, which may be connected by means of a DC. path in serieswith the signal drive circuit for one of the driver circuit grids. Thisis equivalent to stating that feed-back signal is con- .I Iected inseries with input signal. The magnitude of the feed-back voltage whichmay be provided, without causing any trace of instability is greaterthan 36 db. How- .ever, since the driving circuit voltage required underthese conditions is too great, the circuit is normally employed withabout 24 db of feed-back. As a variant signal input may be applied toone grid of the driver and feed-back voltage to the other, each insuitable phase relation.

The feed-back winding is very closely coupled to the normal outputtransformer secondary winding, but is electrostatically shielded fromthe latter. Thereby, the sole energy coupled into the feed-back windingis that due to magnetic coupling. It has been found that the amount offeed-back which can be employed is greatly reduced, if theelectro-static shield is eliminated, because currents transferred to thefeed-back winding capacitively may be of such phase as to tend to causeinstability, and are a function of frequency. The feed-back circuit, inaccordance with the present invention, looking from the primary windingof the output transformer, to the signal input grids of the poweramplifier, includes no capacitive element, at any point in the feed-backloop, so that there is less tendency toward instability, even forextreme values of feed-back voltage.

Amplifiers constructed in accordance with the teachings of the presentinvention have been found to be remarkably insensitive to supply voltagevariations. For example, when operated with well regulated'screen andplate supply voltages, the residual hum in the output was 96 db below 50watts. When a ripple voltage of 42 volts was inserted in series with theplate supply, or 9 volts in series with the screen supply, the residualhum increased to only db below 50 watts.

On the basis that input signal is set to the value necessary to produce2 percent distortion in the output, and employing a pair of 1614 typetubes in the power amplifier, about 60 watts of output may be deliveredby the system, in the range 40 to 4000 c.p.'s. At the lower frequenciesoutput is limited by the inability of the tubes to supply adequatemagnetizing current. In the intermediate range output is limited by peakclipping due to the inability of the driver tubes to drive the grids ofthe power tubes positive, and at the high frequencies output power islimited by the inability of the power tubes to supply the chargingcurrent required by the inter-winding capacity. At the low end of therange, the amplifier has a drop-off rate of.9 db/octave while at thehigh end it approaches a drop-off rate of 6 db/octave, which iscompletely adequate for high fidelity audio operation.

Most of the power in speech, song, and music is contamed in thefundamental tones, with frequencies below 3000 cycles/ sec. The powerlevels of the higher frequency fundamental tones and of the harmonics ofthe lower frequency fundamentals drop off at a greater rate than thepower-delivering capacity of the present amplifier. It results,therefore, that the power-delivering capacity of the amplifier is fullyadequate for all audio frequency signals. Test results show that theamplifier is capable of developing its full power output of 60 wattsover most of the middle frequency range with total plate circuit losses,including transformer losses, considerably lower than rated values. Incertain tests, the screen dissipation exceeded the rated value by about7 percent, but only when the amplifier was being over-driven, to obtaina 2 percent distortion level. A reduction of 3 watts in the outputpower, over most of the range, brings the harmonic distortion below 1percent, and the screen dissipation safely below the rated value. Themaximum plate circuit efficiency occurs in the 500-1000 cycle/ sec.range and is 65.2 percent. This value includes the output transformerlosses, and is remarkably close to the ideal or theoretical value of78.5 percent for class B operation, which does not include transformerlosses.

The power supply construction required for the amplifier of the presentinvention may be extremely simple. For example, filter chokes are notnecessary in either the plate or the screen supply circuits. A singletype 5U4-G rectifier tube, operating within manufacturers ratings, isadequate to supply the plate circuits of the power tubes, due to thehigh efficiency of operation of the latter. A single 6X4 type rectifiertube supplies the power required by a pre-amplifier and the screens ofthe power tubes, and a single 6X4 type rectifier supplies the negativevoltage required by the driver.

The pre-amplifier employed consists of a two-stageresistance-capacitance coupled amplifier, the first stage of whichincludes an un-bypassed relatively low cathode re sistance, and aninternal feed-back loop is provided between the second plate and thefirst cathode, which establishes good wave shape and low outputimpedance. The pre-amplifier is coupled to the driver grid by means of acondenser and choke coil in series, having low D.C. resistance and a lowQ resonance between 10 and 15 cycles per second. The low D.C. resistanceis essential to prevent driver grid current changing the bias relationsof the phase inverter. Feed-back from the output transformer secondaryis also incorporated, from a tap of the secondary output to the firstcathode of the preamplifier, the coupling circuit including a simpleseries capacitor. This capacitor has no observable effect on lowfrequency response, and is found to reduce, the tendency of theamplifier to ring with sharp rise time square wave inputs. More complexcircuit such as bridged-T networks may be used for the same purpo'e,since the time constant of this feed-back circuit may be made extremelylow.

In order to isolate the direct drive of the inverter stage from the mainfeed-back drive, while retaining a common point of reference for both inthe system, the former may drive one of the tubes of the inverter andthe latter may drive the other. The amplifier then includes threeseparate feed-back loops, a first extending from the output transformerto that side of the phase inverter driver stage which is not suppliedwith signal input, a second extending from the output transformer to thecathode of the first stage of the pre-amplifier, and a third extendingfrom the second anode of the pre-amplifier to the first cathode of thepro-amplifier.

It is accordingly, a broad object of the present invention to provide anovel amplifier capable of high power output over a wide frequency band.

It is another object of the invention to provide a novel amplifier ofthe class B type, which utilizes a pair of vacuum tube devices connectedin push-pull relation, which operate with cathodes grounded.

It is a further object of the present invention to provide a novel classB power amplifier which provides high power output, at greater than 50percent efficiency, with relatively low distortion, over a relativelywide band of frequencies.

Another object of the invention resides in the provision of a novelnegative feed-back system for a push-pull power amplifier, whichincludes a statically shielded winding in the output transformer of theamplifier.

Still another object of the invention resides in the provision of anovel negative feed-back system for a power amplifier, including astatically shielded secondary winding in the output transformer, and aphase inverter driver, directly coupled to the power amplifier, in anegative feed-back loop.

A further object of the invention resides in the provision of a novelpro-amplifier having provision for internal negative feed-back, and forthe introduction of a further negative feed-back voltage via a feed-backloop extending from the output secondary winding of the outputtransformer of a power amplifier which is supplied with signal by thepro-amplifier.

Still a further object of the invention resides in the provision of anovel power amplifier which is substantially free of hum output due topower supply hum input of considerable amplitude, enabling utilizationof a relatively inexpensive filter in the power supply of the systern.

It is a further object of the present invention to provide a novel highfidelity amplifier which employs a pushpull output transformer havingbi-filarly related primary halves, in which the transformer is designedfor minimum interwinding capacitance.

Another object of the invention resides in the provision of a novelpush-pull power output transformer having bi-filarly wound primaryhalves and having provision for reducing the capacitance between theprimary halves without introducing thereby appreciable leakagereactance.

It is a further object of the invention to provide a novel powertransformer having bi-filar windings, in which the adjacent conductorsof the bi-filar windings are periodically transposed in order to reduceinter-winding capacitance. J f

It is a further object of: the present invention to provide a noveltransformer employing bi-filar windings, in which the bi-filar windingsare transposed at random, to reduce inter-winding capacitance.

The above and still further features, advantages and objects of theinvention will become apparent upon consideration of the followingdetailed description of a specific embodiment thereof, especially whentaken in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a simplified schematic circuit diagram of a power amplifierand driver, arranged in accordance with the present invention;

FIGURE 2 is a schematic circuit diagram of a' complete amplifier inaccordance with the invention, including power rectifiers,pre-amplifier, power amplifier and driver;

FIGURE 3 is a view in cross section taken through the windings of theoutput transformer of FIGURES 1 and 2, illustrating the mechanicalarrangement and the winding buildup;

FIGURE 4 is a partial view in cross section taken through the windingsof an output transformer alternative to that of FIGURE 3;

FIGURE 5 is a partial view in cross section of a further variation inthe windings of an output transformer in accordance with my invention;and

FIGURE 6 is a series of curves illustrating the performance oftheamplifier of FIGURE 2.

Referring now more particularly to FIGURE 1 of the accompanyingdrawings, the reference numeral 11 denotes a twin triode driver stage ofthe phase inverter type, which drives a push-pull power amplifier 12 ofthe class B type. The power amplifier 12 comprises a pair of vacuumtubes 13, 14, which may be of one of the types commercially known by thetype designations 6L6, 1614 or 807 for example only. The cathodes of theamplifier tubes 13, 14 are connected directly to a point of referencepotential,conventionally represented at ground, 15. The anodes of thetubes 13, 14 are connected respectively in series with transformerprimary halves 16, 17, a common point 18 of which is connected to asource of positive voltage represented by the terminal 19'.

The vacuum tubes 13, 14 may be of the tetrode type, and morespecifically may be of the beam power type. The screen grids of thevacuum tubes 13, 14 are connected jointly to a source of screen voltagerepresented by the terminal 20. As will appear hereinafter, the sourcesof anode and screen voltages employed are not critical, and need not beseverely filtered to remove voltage at power frequency, nor need thesevoltages be regulated. Recommended values for one of the recommendedtube r types, noted for the sake of example only, are 550 v.

anode voltage and 350 v. screen voltage.

The driver stage 11 comprises a twin triode vacuum tube, 21, which maybe of the 12AX7 type, for example, and have triode sections 22, 23. Thecathodes of the triode sections 22, 23 are connected together and via aresistance 24 to a source of negative voltage, represented by theterminal 25. The latter may have a value of l v., in one specific designof the present system. The anodes of the triode sections 22, 23 areseparately connected via anode load resistances 26, 27 to the junctionof two resistances 28, 29, which extend in. series between the terminal20 and the ground point 15. The resistances 28, 29 in series constitutea voltage divider, and may have values of approximately K and 5K,respectively, so that the voltage with respect to ground which issupplied to the anode circuits of the triode sections 22, 23 is of theorder of 10 v. This relatively low value of anode supply voltage is, ofcourse, feasible because the cathodes of the triode sections 22, 23 areoperated at a relatively high negative voltage. v

The anodes of the triode sections 22, 23 are connected directly with thecontrolelectrodes of the power amplifier tubes 13, 14, respectively,i.e. without the interposition of capacitance so that the steadyvoltages on the anodes of the triode sections 22, 23 establish thecontrol electrode bias voltages for the power amplifier tubes 13, 14.The bias voltage may, by selection of voltage supply values, operatingconditions and circuit parameters for the triode sections 22, 23, beestablished at a value suitable for class B operation of the poweramplifier tubes 13, 14, or at some other value which establishes highefiiciency opera- 'tion. More specifically, a quiescent anode currentfor the power amplifier tubes 13, 14 may be established at about 15 ma.,although I do not desire to be limited to any specific value.

The bias voltage for the control electrode of the triode section 23 maybe established by a voltage divider, comprising resistances 30, 31connected in series between the ground point 15 and the negative voltageterminal 25. The control electrode of triode section 23 is connected tothe junction 32 of the resistances 30, 31, and the values for theseresistances are selected to establish a bias voltage suitable for classA operation. The junction 32 is connected directly with one inputterminal 33 of the amplifier. The remaining input terminal 34 may beconnected directly to the control electrode of the triode section 22,and if it be assumed that the terminals 33, 34 are connected to thesecondary windings of a signal input transformer 35, the control gridsof triode sections 22, 23 are driven at least approximately equally.

. 'In operation, input signal applied at terminals 33, 34 varies thevoltage of the control electrode of triode section 22 about thequiescent value established by the voltage divider 30. The cathode ofthe triode sections'22, 23 follow the resultant current variations inthe triode section 22, and transfer an effective out-of-phase signalvoltage between the cathode and control electrode of the triode section23. The triode sections 22, 23 thus operate as a phase splitter suitableboth for establishing bias for, and driving, the power tubes 13, 14, andthe source of screen voltage for the power tubes, 13, 14, is utilized asan anode source for the triode sections 22, 23.

. The windings 16, 17 of the output transformer T are bi-filarlyrelated, and the design of the transformer T is an important feature ofthe present invention. The effect of the transformer design on amplifieroperation will be discussed hereinafter. For the present, it is notedthat the transformer T includes an output secondary winding '37, whichis not directly tied to ground. The feed-back winding 37 is staticallyshielded by means of a static shield 38, connected to a point ofreference potential, i.e. ground. The feed-back secondary winding 37 maybe connected, via leads 38a, in series with the secondary winding of asignal input transformer 35, by breaking the lead between the terminal34 and the control electrode of the triode section 22, and thenconnecting leads 38a to the break points respectively, as at 39. It maybe noted that the feed-back loop constitutes a DC. path.

The utilization of a shield 38 about the feed-back winding 37 eliminatesvoltages from that winding which might otherwise be present, due tocapacitive coupling otherwise existent between the feed-back winding andthe other windings of the transformer. Such voltages may be ofconsiderable magnitude, in a transformer operating at high voltage andpower, and may be of improper phase to provide degeneration, but insteadmay lead to instability. The fact that only induced voltages due totransformer magnetic flux variations exist in the secondary winding 37,and that the entire feed-back loop from feed-back winding 37 to theinput of power amplifier 12 includes no capacitive circuit elements,enables use of a tremendous value of feed-back, i.e. about 36 db. Muchless than this value of feed-back may be employed in practice, in orderto reduce the required input signal, and since the stated value is foundto be greater than is required in practical designs, and about 24 db offeed-back is recommended in practice. Nevertheless, the overall designof the amplifier, by enabling use of such large values of feed-back,permits also a considerable simplification of power supply system, andtherefore of overall cost.

The power sources connected to the terminals 19,

and 25, respectively are in no sense critical, and need not beregulated, nor severaly filtered, and in fact very heavy ripple voltagesof the order of 42 volts in the anode sup- .ply and 9 vvolts in thescreen supply may be tolerated,

8 and produce no appreciable output signal. It is this fact whichenables the same power terminal 20 to be employed to supply screenvoltage and bias to the power tubes 13, 14 and anode supply voltage forthe triode sections 22, 23.

The elimination of coupling and DC. isolating capacitors for theamplifier circuit proper also eliminates the possibility of blocking ofthe amplifier during transient overloads, and in general contributes toexcellence of transient response of the amplifier.

In general, it is not desirable to supply the input signal to a poweramplifier by means of an input transformer as in FIGURE 1 of theaccompanying drawings, and the problem therefore exists of driving thedriver stage 11 from a pre-amplifier which is coupled with the input ofthe driver stage otherwise than by means of a transformer. Thetransformer 35 is, therefore, included in the circuit of FIGURE 1 in theinterest of simplicity of illustration and exposition.

Reference is now made more particularly to FIGURE 2 of the accompanyingdrawings, which illustrates a modification of the amplifier illustratedin FIGURE 1 of the accompanying drawings, together with a suitablepreamplifier, and rectifier power supplied.

Considering first the preamplifier arrangement, there is employed a pairof cascaded triodes 40, 41, although other suitable vacuum tubes such astetrodes or pentodes may be employed. Input signal is applied between aterminal 42 and a point of reference potential, 43, which may be ground(as is point 15), across a variable volume control potentiometer 44. Thevariable contact of the potentiometer 44 is connected directly to thecontrol electrode of the triode 40. The cathode of the latter isconnected to the reference point 43 via an unby-passed resistance 45 andits anode is plate loaded by a resistor 40a supplied from the terminal20, via a suitable voltage divider consisting of resistance 47, andresistance 48 connected in series to reference point 15. Anode potentialis taken from the junction of resistances 46 and 47, and is supplied tothe anode load resistor 40a in series with a resistance 49. A filtercapacitor 50 is connected from the high potential end of resistance 40ato the reference point 43.

The signal voltage present at the anode of triode 40 is transferred tothe control electrode of the triode 41 via a coupling capacitor 51, thecircuit of triode 41 includ ing a cathode resistance 52, lay-passed by acapacitor 53, so that resistance 52 acts only to supply bias. The triode41 is anode loaded by a resistance 41a, and supplied with anode voltagefrom the junction of resistances 47 and 49. This anode voltage isfiltered by a capacitor 55. An internal feed-back loop exists betweenthe anode of triode 41 and the cathode of triode 40, consisting of acapacitor 56a and a resistance 57 connected in series. A further overallfeed-back loop exists, in the form of a connection from a tap 56 onsecondary winding 36, via capacitor 58, to the cathode of triode 40. Atthe same time distortion introduced into the transformer output power bythe preamplifier may be eliminated, by the negative feed-back. It is animportant feature of the preamplifier design that the same point, i.e.the cathode of triode 40, may be utilized for both internal and externalnegative feed-back voltage insertion.

The output of the preamplifier is supplied to the control electrode ofthe triode section 22 by means of a capacitor 60 and choke coil 61,extending in series from the anode of the triode '41 to a point on avoltage divider which establishes the bias for the triode sections 22,and 23. This voltage divider includes three resistances, 62, 63, 64,connected in series from terminal 25 to the reference point 15. :It willbe recalled that the common cathode load 24 for the cathodes of triodesections 22, 23 extends to the negative supply terminal 25. The feedbackwinding 37 is connected between the control electrode of the triodesection 22 and the-junction of ca- 9 pacitor '60 and choke coil 61. Thecontrol electrode of triode 23 is connected to the junction betweenresistances 62 and 63, and the choke 61 extends from the capacitor 60 tothe junction of resistances 63 and 64.

The capacitor 60 and the choke 61 have a low Q resonance at 10 orcycles, i.e. below the pass band of the amplifier, and provide anextremely low resistance coupling circuit, of relatively high inductivereactance. This is of importance because the triode section 23 drawsgrid current when its grid voltage becomes more positive than 1 vol-t,and this grid current must not be allowed to upset the bias relations inthe phase inverter 21.

I now turn to consideration of the output transformer of the presentinvention, its construction, and arrangement and the designconsideration affecting these.

It is well known to use bi-filar output transformers, in wide band audiopower amplifier systems intended for high fidelity operation. See, forexample, the article by F. H. McIntosh and G. T. Gow, entitled,Description and Analysis of a New SO-Watt Amplifier Circuit, publishedin Audio Engineering, December 1949. However, the McIntosh circuit doesnot operate with the cathodes of its power amplifier tubes at ground orat the same fixed reference potential, and the design of the McIntoshcircuit is such that certain problems of charging transformercapacitance are avoided, which cannot be avoided if the cathodes are tobe grounded. In the present system it is desired to utilize amplifiertubes, in the power stage of the transformer which operate with cathodesat ground, or at fixed reference potential, because this makes possibleuse of a relatively inexpensive power supply system. The power outputtubes are plate loaded only, and the two primary windings of thetransformer are bi-filarly related in order to reduce leakage reactanceand thereby to improve performance. In particular this design serves toeliminate the conduct-ion transfer notch, due to leakage reactance, andwhich has been a serious problem in output transformer design, in classB amplifier.

The use of bi-filar windings in the present system introduced newproblems which were not previously important. One of these problems isthat appreciable peak voltage may be required to exist between adjacentwires of a bi-filar winding, and these voltages cannot exist unless thewinding-to-winding capacitance has been charged. The charging currentmust usually be supplied through the output power tubes, and may be amajor factor in limiting the high frequency power-delivering capacity ofthe amplifier.

It can be shown, in particular, for the arrangement illustrated in thecirciuts of FIGURES 1 and 2, that the peak voltage existing from end toend of one primary coil is also half the voltage existing between allcorresponding or adjacent points of the two primary windings. Thisvoltage may be about 500 v., and the capacitance involved may be about.045 microfarad, in the absence of special transformer features. A peakcharging current of 1.5 amperes is required to charge the statedcapacity to the stated voltage, assuming a sine wave at 10 kc., and thisis beyond the power delivering capacity of the tubes involved. Attemptshave been made to reduce the capacity of the windings by sectionalizingthe windings, and adopting various section interconnections. These havenot succeeded, in general, and have introduced new problems ofinterwinding insulation, and the like, by introducing high voltages atcertain portions of the winding.

In the general case of two isolated paralleled circular wires anincrease of the spacing of the wires, from 10 percent to 100 percent ofthe diameter of the wire, will reduce capacitance between wires byapproximately 70 percent. If the adjacent surfaces are separated by onlyfrom 10 percent to percent of the wire diameter, a decrease of interwirecapacity of about 30 percent may be attained.

It is true that in a transformer one does not deal with two paralleladjacent wires, but thegeneral principle remains applicable. Eachconductor of the transformer will have capacitance to the wires oneither side thereof, and to the Wires in the layers above and below it.The capacitance between wires in the same layer may be reduced 50percent by transposing the two wires of the bi filar winding at everyturn. A somewhat smaller reduction may be attained by randomtransposition. The capacitance between adjacent layers is not reduced inthis manner. 'In the non-transposed winding, assuming the same spacingbetween layer centers which exists between adjacent wire centers in alayer, and assuming uniform dielectric material, the capacitance betweenthe wires in the same layerv accounts for about two-thirds of the totalcapacitance, and the capacitance between adjacent layers accounts forthe remaining one-third. Since the transposition of wires can beexpected to cut in half the capacitance between wires in the same layer,this expedient reduces the total capacitance of the windings by a factorof one-third.

The capacitance between adjacent layers may be reduced by increasing thespacing between the layers, at the cost of some increase of leakagereactance. However, an increase of this spacing leaves the transformerbi-filarly wound, and it has been found in practice, that the resultantincrease in leakage reactance is not such that the conduction transfernotch appears.

The transformer winding which was evolved in accordance with theprinciples of the present invention was designed to be employed withtwo-grain-oriented Hipersil cores Moloney ME-31 Hipercores, orequivalent) and FIGURE 3 of the accompanying drawings illustrates thecoil buildup, being proportioned to scale vertically but nothorizontally. Various transformers, constructed in accordance with theprinciples of the present invention, and tested for capacitance, showedan inter-primary capacitance of about .01 microfarad. This low value isattained directly by (1) transposing alternate turns within each layer,(2) spacing adjacent layers sufliciently, and indirectly by (3)employing grain-oriented cores. It has been found that the use ofnon-grain-oriented cores renders it necessary, for the same lowfrequency standard of performance, to increase the core cross sectionand the number of turns by about 25 percent, and the combined effectincreasing primary interwinding capacitance by about 50 percent.

Turning now more particularly to FIGURE 3 of the accompanying drawings,the reference numeral 70 represents a cross section taken through onewall of a cylindrical coil form, designed to fit two Moloney ME-31Hipercores, or equivalent, and to provide insulation for 1000 voltsbetween coil and core. Wound on the coil form 70 is a first, or innerlayer of bi-filar winding 71, comprised of fifty bi-filar turns of No.28 Formvar double cotton covered wire, one of the wires 72 of eachbi-filar pair being indicated as 1, and the other 73, as 2. It will beclear that each turn has its wires 1 and 2 transposed, i.e. wire 1appears first to the left and then to the right of wire 2, in alternateturns of each layer, so that this alternation of transposition continuesacross the coil. In all six such layers are employed, and adjacentlayers are separated from one another by two layers 74 of .005 kraftpaper, except centrally of the coil buildup, where certain secondarywindings are inserted. After the first six layers 71 have been wound, acovering 75 of five layers of .005" kraft paper are laid thereon, and onthe covering 75 is wound two secondary windings 76 and 77- in the samelayer, the secondary winding 76 consisting of twelve (12) turns of No.16 F. V. insulated wire, and the secondary winding 77 of twenty-nine(29) similar turns. The covering 78 may be provided for the secondarywindings 76 and 77, consisting of three layers of .005" kraft paper, anda third secondary winding 78a may be wound on the latter, consisting ofseventeen (17) 11 turns of No. 18 F. V. insulated wire, and extendingabout half the axial length of the coil.

Also superposed on the covering 78 is a layer 79, of metallic or otherhighly electrically conducting material, extending laterally a littleless than half the length of the coil. The conducting layer 79 iscovered by two layers 80 of .005" kraft paper, and on the latter iswound forty (40) turns of No. 32 F. V. insulated wire, in a singlelayer. The latter is covered by two layers 82 of .005" kraft paper, andsuperposed on the latter is another layer of highly conducting material83, such as metal or the like. A heavy covering 84 of insulating spacer,consisting of five layers of .005" kraft paper is then placed about thesecondary Winding 79 and about the outer conductive layer 83. Thedimensions of the wire and coverings interposed between the papercoverings 78 and 84 are such that the layer 84 may lie fiat and true.

Superposed on the covering 84 are six additional layers 85, of bi-filarwinding, similar in arrangement to the layers 71, i.e. with the wiretransposed after each turn, and with the successive layers separated bytwo layers 86 of .005" kraft paper. The entire coil buildup may then bewrapped with outside insulation 87, of suitable character.

The winding 81 constitutes the statically shielded feedback winding 37of FIGURE 1, and the separate conductors of the six layer bi-filarwindings constitute the primary halves of FIGURES 1 and 2. The severalsecondary windings 76, 77, 78a, may be interconnected in series, andtaps brought out from the junction points, to provide output windings ofvarious elfective impedances. 'To be noted are the relativelysymmetrical location of the feed-back winding 81, between the primarywinding halves, and the fact that alternate turns of the latter aretransposed. The shield 79, 83, is the basic element which serves toisolate the feed-back winding statically, so that the total feed-backvoltage is that induced magnetically, in response to current flow in theprimary windings. Further, the shield 79, 83 would, of course,accomplish this object of itself, were the windings not transposed. Thefeed-back winding 81, being adjacent the several secondary windings ismore closely coupled to the latter than might otherwise be the case, andhence responds to output current delivered by the transformer. Thisincreases the accuracy of feed-back in some degree.

As mentioned in the above general discussion pertaining to the design ofa transformer in accordance with my invention, a reduction incapacitance between wires in the same layer may be attained by randomtransposition. Examples of such transposition are illustrated in FIGURES4 and 5.

Referring to FIGURE 4, a portion of a winding similar to thatillustrated in FIGURE 3 is shown, except that the transposition of thewires within a given layer of the winding is carried out in a randommanner. The randomness may be extended throughout the primary windingand it will be noted that elements in FIGURE 4 are numbered the same asthe corresponding elements of FIGURE 3. It will be obvious that suchrandomness could be carried on into the outer section of the primarywinding.

Good results may also be achieved by using a random (layerless) bi-filarwinding. FIGURE 5 shows a winding of this type, with the addition of alow-dielectric-constant, non-conducting fiber or filament designated as3. This filament is wound along with the bi-filar elements 1 and 2 andis for the purpose of controlling the interwinding capacitance.

The diameter of the fiber 3 may be of any suitable size comparable tothe diameter of the wires 1 and 2, and it is determined by a compromisebetween the size of the overall winding and the extent to which theinterwinding capacitance is to be reduced. As is the usual practice inproducing a layerless winding, a coil form such as that designated at70a is required to retain the 12 wires at the ends of the winding. Theouter section of the primary may be wound in a similar manner, thesecondary and feed-back windings being similar to those of FIGURE 3.

The examples of windings illustrated in FIGURES 3, 4, and 5 are merelyillustrative of the variations which a transformer in accordance with myinvention may assume.

Returning now to FIGURE 2 of the accompanying drawings, the power supplyfor the present system includes a transformer 90, having a primarywinding 91 and a plurality of secondary windings 92, 93, 94. The centertap of transformer secondary winding 93 is connected to ground point 15.Also connected to point 15 is one end of the secondary winding 94, whichmay supply current to various heaters marked X. The secondary winding 92is connected at one end to the high voltage line 95, which supplies theamplifier tubes 13 and 14, and supplies power to a filament 96 of adouble diode rectifier tube 97. The anodes of the rectifier tubes 97 areconnected to points of opposite phase and equal amplitude of thesecondary winding 93. The rectifier tube 97, which may be of the 5U4type, has the sole function of supplying power to the anodes of thepower amplifier tubes 13, 14, via lead 95.

A further double diode rectifier tube 98 is connected at its anodes topoints of opposite phase of the transformer secondary Winding 93, andsupplies positive screen voltage at its cathode for the power amplifiertubes 13, 14, via lead 99, and anode voltage for the twin triode 21 vialead 100. This same lead delivers voltage to the pre-amplifier anodes oftriodes 40 and 41. The double diode 98 may be of the 6X4 type.

A tap 101 is taken from the transformer secondary winding 93, forconnection via lead 102 jointly to one anode 103 of double diode 98 andto the cathode of a rectifier tube 104. The anodes of the rectifier tube104 are connected together and via a resistor 105 to point 25,

which is in turn connected via cathode load 24 to the cathodes of thetwin triode 21. The rectifier tube 104, then, supplies negative voltageto the point 25.

It will be noted that the power supply is not regulated, and does notinclude any choke filters, but only capacitive filters. So the RC filter106 filters voltage on the lead 95, a single filter capacitor 107 isemployed to filter the voltage on lead 99, and the capacitors 108 and109 serve to filter the negative voltage supplied to point 25. The useof relatively slight filtering is made possible by the unresponsivenessof the system to hum voltage in the voltage supply leads, as has beenpointed out hereinbefore, and the fact that a relatively poorly filteredsupply may be employed, as well as the fact that the system operates athigh elficiency, enables an output of about 60 watts to be attained,without overloading the power supply or the output power tubes.

The frequency response curve of the amplifier is illustrated in FIGURE 6of the accompanying drawings, plotted in terms of output level in db forthe frequency range 20 to 200,000 cycles per second, and utilizing 50watts as a reference output level. Curve A shows the output in db overthe frequency range, when input is held constant at .031 volt, theportion A1 being derived when the capacitor 58 is 0, the portion A2being produced when the capacitor 58 is 15 mmfd. and the portion A3being for Gaussian response with minus 3 db at 75 kc. (for comparisonpurposes). Curve B shows the response in db for an input of .35 vol-tover the range 20 to 20,000 cycles per second. Curve C shows the outputwhich results when input is adjusted to a level sufficiently high tointroduce 2 percent distortion in the output. This curve wasexperimentally plotted to 20,000 cycles per second, and extrapolatedbeyond.

It is well known that most of the power in speech, song, and music iscontained in the fundamental tones with frequencies below 3,000 cyclesper second. The power levels of the higher frequency fundamental tonesand of the harmonics of the lower frequency tones decrease rapidly asthe frequencies become greater than 3,000 cycles per second. Studies ofthe power requirements for various frequencies of speech, music, andthelike have been made. On the basis of these studies, conducted inconnection with a plurality of musical instruments and combinationsthereof, curves have been compiled showing the maximum power requirementwhich may be expected at each frequency. On the basis of theserequirements it has been shown that the amplifier of the presentinvention has adequate power delivering capacity at all frequencies,excellent transient response, and excellently low harmonic andinter-modulation distortion, and is capable of delivering about 50 wattsof substantially undistorted power over those portions of the audiofrequency spectrum which require maximum power.

While I have described and illustrated specific embodiments of thepresent invention, it will be clear that variations of arrangement anddetail may be resorted to without departing from the true scope of theinvention as defined in the appended claims. In particular I desire itto be understood that while I have disclosed my invention as applied toa class B or similar high efficiency amplifier, the system may beemployed to advantage in amplifier types operating class AB, or A,subject to consequent reduction of efiiciency.

What I claim is:

1. A wide band audio transformer for use as an output transformer of apower amplifier, said transformer having primary windings on a commoncore consisting of a first primary winding having two ends, and a secondprimary winding having two ends, said first and second primary windingsbeing bi-filarly related, said first primary winding having a firstpositive voltage terminal and a first anode terminal, said secondprimary winding having a second positive voltage terminal and a secondanode terminal, the ends of said first and second primary windings towhich said first anode terminal and said second positive voltageterminal are connected respectively being immediately adjacent, thesecond anode terminal and said first positive voltage terminal beingconnected respectively to the remaining ends of said primary windings,said primary windings consisting of a plurality of turns of bi-filarlyrelated conductor pairs, said turns having a common axis, each conductorpair consisting of a first conductor and a second conductor, saidplurality of turns being arranged to include at least portions havingone relative orientation of the first and second conductors with respectto a predetermined direction along said axis and at least other portionshaving an opposite relative orientation of the first and secondconductors with respect to said predetermined direction along said axis,at least portions of the second conductors of different turns beingimmediately adjacent each other and at least portions of the firstconductors of different turns being immediately adjacent each other byvirtue of said relative orientations of said first and second conductorswith respect to said predetermined direction along said axis.

2. A transformer according to claim 1 wherein said primary windingsoccupy successive layers of winding of said transformer, and insulatingsheet material separating the layers and substantially separating saidlayers by a distance at least of layer thickness.

3. A transformer according to claim 1 wherein said primary windings arelayerless windings having a total thickness perpendicular to theirwin-ding sense which is greater by a factor of at least five than thethickness of the conductors constituting the windings.

4. The combination according to claim 1 wherein is further provided asecondary feed-back Winding on said core, and means for staticallyshielding said secondary feed back winding from said primary windings,said secondary winding being at least one layer of winding on said corecontained within the confines of said primary windings, said means forshielding consisting of flexible metallic sheetmaterialwound bothinternally and externally of said at least one layer of windingand'located to statically shield said secondary winding from all theprimary winding turns.

5. A wide band audio'transformer for-use as an output transformer of apower amplifier, said transformer having primary windings on a commoncore consisting of a first primary winding having two ends, a secondprimary winding having two ends, said first and second primary windingsbeing bi-filarly related, said bi-filarly related primary windings beingwound in a plurality of superposed multi-turn layers of bi-filarwindings with the layers each including a large plurality oftranspositions of the relative positions of the conductors of thebi-filar winding, said first primary winding having a first positivevoltage terminal and a first anode terminal, said second primary windinghaving a second positive voltage terminal and a second anode terminal,the ends of said first and second primary windings to which said firstanode terminal and said second positive terminal are connected beingrespectively immediately physically adjacent, the second anode terminaland said first positive voltage terminal being connected respectively tothe remaining ends of said primary windings, and a secondary outputwinding substantially symmetrically positioned among said layers.

6. A Wide band audio transformer for use as an output transformer of apower amplifier, said transformer having primary windings on a commoncore consisting of a first primary winding having two ends, a secondprimary winding having two ends, said first and second primary windingsbeing bi-filarly related, said bi-filarly related primary windings beingwound in a plurality of superposed multi-turn layers of bi filarwindings with the layers each including a large plurality oftranspositions of the relative positions of the conductors of thebi-filar winding, said first primary winding having a first positivevoltage terminal and a first anode terminal, said second primary windinghaving a second positive voltage terminal and a second anode terminal,the ends of said first and second primary windings to which said firstanode terminal and said second positive terminal are connected beingrespectively immediately physically adjacent, the second anode terminaland said first positive voltage terminal being connected respectively tothe remaining ends of 'said primary windings, a secondary feed-backwinding layer substantially symmetrically interposel between a pair ofadjacent ones of said multi-turn layers, and means for staticallyshielding said secondary feed-iback winding comprising highly conductivematerial disposed Intermediate said feed-back winding layer andmulti-turn layers immediately adjacent to said feed-back winding layer.

7. A wide band audio transformer for use as an output transformer of apower amplifier, said transformer having primary windings on a commoncore consisting of a first primary winding having two ends, a secondprimary winding having two ends, said first and second primary windingsbeing bi-filarly related, said bi-filarly related primary windings beingwound in a plurality of superposed multi-turn layers of bi-filarwindings, with the layers each including a large plurality oftranspositions of the relative positions of the conductors of thebi-filar winding, said first primary winding having a first positivevoltage terminal and a first anode terminal, said second primary windinghaving a second positive voltage terminal and a second anode terminal,the ends of said first and second primary windings to which said firstanode terminal and said second positive terminal are connected beingrespectively immediately physically adjacent, the second anode terminaland said first positive voltage terminal being connected respectively tothe remain-ing ends of said primary windings, a secondary feed-backwinding, and means for statically shielding said secondary feed-backwinding from said primary windings.

References Cited in the file of this patent UNITED STATES PATENTS Franket a1 Ian. 8, 1918 Bowman Nov. 4, 1919 Pratt Dec. 14, 1920 Farry Feb. 9,1943 Great Britain Mar. 25, 1953 p 1 na-r

